The present invention relates to a method of producing a two-dimensional material particularly but not exclusively graphene and silicene. The invention also relates to a method of producing heterostructures comprising two-dimensional materials.
Graphene is a well-known material with a plethora of proposed applications driven by the material's theoretical extraordinary properties. Good examples of such properties and applications are detailed in ‘The Rise of Graphene’ by A. K. Geim and K. S. Novoselev, Nature Materials, vol. 6, March 2007, 183-191.
Nevertheless, to achieve these desired material properties and applications, it is well understood that it is essential for the graphene to have a number of characteristics including:
i. Very good crystal quality, namely that the graphene structure lattice is extremely uniform about all axes, highly repeatable in symmetry throughout the full monolayer and exhibits minimal lattice malformations;
ii. Large material grain size, whereby the grain structure of grown graphene exhibits individual grain dimensions ≥10 μm×10 μm;
iii. Minimal material defects, whereby defects include crystal lattice breaks, interruptions, atomic or molecular contamination to the crystal from other elements, or poor graphene monolayer surface condition through, for example oxidation;
iv. Large sheet size, i.e. greater than 3 cm×3 cm and preferably in the order of 10 s of centimeters; and
v. Be self supporting so that a complete sheet of the size given in iv above can be removed intact from the substrate upon which it was produced.
Conventional graphene production methodologies to date have been unable to manufacture graphene with all of the aforementioned properties. Consequently, the predicted performance properties and device applications of graphene have yet to be realized.
Several conventional methods of graphene production exist and are in wide use;
examples are described in:
US 20130156678 A1—solution based electrophoresis of graphene on metallic substrates or films, whereby an electrical potential is applied to a conductive substrate immersed in carbon comprising solution. The result is carbon transport, due to the applied field, to the substrate surface at which point self alignment of the carbon into graphene ‘sheets’ occurs;
U.S. Pat. No. 8,147,791 B2—a graphene oxide reduction mechanism whereby graphene oxide is introduced to a water and solvent solution and heated to moderate (<300° C.) temperature resulting in the dissociation of oxygen and allowing carbon amalgamation, potentially in graphene crystal structure configuration;
WO 2014110170 A1—a catalysis driven chemical vapor deposition (CVD) technique whereby a heated copper substrate is used as a catalytic surface in a standard CVD chamber for the decomposition of hydrocarbons resulting in carbon being left on the metallic surface.
Beyond the current inability to achieve the important material properties described above, further limitations exist to these conventional techniques.
The need of special sacrificial metal catalyst substrates to promote graphene formation introduces limitations to the production process parameters. Examples of these limitations include the need to use temperatures that do not affect phase changes in the metal substrates and the need to use non-reducing gases and precursors that will not degrade the metallic substrate surface. This inflexibility in production variables has lead, in certain processes, to difficulty to achieve good graphene growth and the inability to remove unintended contamination or dopants.
In current processes, the graphene grains formed are not sufficiently bonded to maintain sheet form once removed from the substrate. Consequently, graphene is most commonly available in the form of flakes or powder, or sheets that comprise a protective fixant that retains the grains together. The protective fixant renders the sheet unsuitable for electronic device construction.
Two further problems with the conventional production methodologies derive from the need to:
a) remove the produced graphene from the equipment to make an electronic device, thus exposing the graphene to the external environment resulting in surface contamination that adversely affects the further processes required to produce the electronic device; and
b) separation of the graphene from the catalyst metal base substrate which requires chemical or physical processes that contaminate the graphene material.
Since the proposal of graphene, additional two dimensional (2D) layers, often termed single layer materials, have been speculated and are now being researched extensively at a rate that now exceeds new research into graphene. Such materials include Silicene, Phosphorene, Borophene, Germanene and Graphyne allotropes of Silicon, Phosphor, Boron, Germanium and Carbon respectively. As with graphene these materials will theoretically exhibit extraordinary properties particularly suited to next generation electronics as outlined in “Electronics based on two-dimensional materials”, Nature Nanotechnology, 2014 (9), 768-779.
In all cases, the realization and hence efficient manufacturing of these materials still remains theoretical although several approaches have been potentially identified, as described in ‘Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene’ (ACS Nano, 2013, 7 (4), 2898-2926. The issues are largely similar to those detailed above for graphene production, however, other 2D materials, unlike graphene, are inherently unstable in air which demands production in inert environments.
Until now, no techniques exist, beyond delaminating single monolayers from bulk materials in nitrogen environments, to produce other 2D materials and no single layers or structures have been successfully produced that survive outside of the controlled inert environment.
The main target applications for 2D materials lie with the combination of these monolayers with semiconductor or dielectric materials in electronic and photonic structures and devices. A plethora of potential inventions have been theorized and speculated, as well detailed in “Science and Technology Roadmap for Graphene, Related Two-Dimensional Crystals, and hybrid systems”, Nanoscale 11, 2015.
Several prototype structures have been realized physically through the manual combination of very small samples, less than 1 cm2, of individual graphene and semiconductor material samples, however the performance of these structures is well below the predicted properties due to the poor quality of the graphene), the manual combination techniques and the inherent contamination that occurs due to the assembly procedure.
The present invention was conceived with the aim of overcoming or at least ameliorating the above problems.